use |f| consistently in README for form builder variable name
use |f| consistently in README for form builder variable name

mikz/formtastic,Davidzhu001/formtastic,nishaya/formtastic,sideci-sample/sideci-sample-formtastic,crashlytics/formtastic,nishaya/formtastic,ConferenceCloud/formtastic,justinfrench/formtastic,Baltazore/formtastic,ryanfox1985/formtastic,Guestfolio/formtastic,ryanfox1985/formtastic,paulodiovani/formtastic,ryanfox1985/formtastic,Davidzhu001/formtastic,justinfrench/formtastic,thenthen8515/formtastic,mikz/formtastic,justinfrench/formtastic,Guestfolio/formtastic,paulodiovani/formtastic,thenthen8515/formtastic,ConferenceCloud/formtastic,EndingChaung/formtastic,EndingChaung/formtastic